Connected: An Internet Encyclopedia
RFC 1630 - 4.2.11. Content-Id

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### 4.2.11. Content-Id

The second scheme, "cid:", is similar to "mid:", but makes reference to a body part of a MIME message by the value of its content-id field. This allows, for example, a master document being the first part of a multipart/related MIME message to refer to component parts which are transferred in the same message.

Note

  Beware however, that content identifiers are only required to be unique within the context of a given MIME message, and so the cid: URL is only meaningful with the context the same MIME message. For a reference outside the message, it would need to be appended to the message-id of the whole message. A syntax for this has not been defined.

Next: 4.2.12. Schemes for Further Study

Connected: An Internet Encyclopedia
RFC 1630 - 4.2.11. Content-Id